The lines are from the epic "Beowulf" which tells of the heroic deeds of the protagonist and hero Beowulf.
Taken from Chapter 23, the lines refer to when the Geats saw the water filled with blood. They did not know that their leader had vanquished and beheaded Grendel and his mother. Rather, they were of the opinion that Beowulf had been killed by the monsters. This is a theme of suspense for the soldiers who are waiting for their leader. They were sure that their master / leader had been killed. Even Hrothgar had turned his boat and sailed, while the Geats waited expectantly and patiently, hoping for Beowulf to return.
